The production of liquid crystal displays, for example, active matrix liquid crystal display devices (AMLCDs) is very complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled.
In the liquid crystal display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred because of their ability to transport electrons more effectively. Poly-crystalline based silicon transistors (p-Si) are characterized as having a higher mobility than those based on amorphous-silicon based transistors (a-Si). This allows the manufacture of smaller and faster transistors, which ultimately produces brighter and faster displays. One problem with p-Si based transistors is that their manufacture requires higher process temperatures than those employed in the manufacture of a-Si transistors. These temperatures range from 450° C. to 600° C. compared to the 350° C. peak temperatures typically employed in the manufacture of a-Si transistors. At these temperatures, most AMLCD glass substrates undergo a process known as compaction. Compaction, also referred to as thermal stability or dimensional change, is an irreversible dimensional change (shrinkage) in the glass substrate due to changes in the glass' fictive temperature. “Fictive temperature” is a concept used to indicate the structural state of a glass. Glass that is cooled quickly from a high temperature is said to have a higher fictive temperature because of the “frozen in” higher temperature structure. Glass that is cooled more slowly, or that is annealed by holding for a time near its annealing point, is said to have a lower fictive temperature. When a glass is held at an elevated temperature, the structure is allowed to relax its structure towards the heat treatment temperature. Since the glass substrate's fictive temperature is almost always above the relevant heat treatment temperatures in thin film transistor (TFT) processes, this structural relaxation causes a decrease in fictive temperature which therefore causes the glass to compact (shrink/densify).
It would be advantageous to minimize the level of compaction in the glass because compaction creates possible alignment issues during the display manufacturing process which in turn results in resolution problems in the finished display.
There are several approaches to minimize compaction in glass. One is to thermally pretreat the glass to create a fictive temperature similar to the one the glass will experience during the p-Si TFT manufacture. There are several difficulties with this approach. First, the multiple heating steps employed during the p-Si TFT manufacture create slightly different fictive temperatures in the glass that cannot be fully compensated for by this pretreatment. Second, the thermal stability of the glass becomes closely linked to the details of the p-Si TFT manufacture, which could mean different pretreatments for different end-users. Finally, pretreatment adds to processing costs and complexity.
Another approach is to increase the anneal point of the glass. Glasses with higher anneal will have a higher fictive temperature and will compact less than when subjected to the elevated temperatures associated with panel manufacture. The challenge with this approach, however, is the production of high annealing point glass that is cost effective. The main factors impacting cost are defects and asset lifetime. Higher anneal point glasses typically employ higher operational temperatures during their manufacture thereby reducing the lifetime of the fixed assets associated with glass manufacture.
Yet another approach involves slowing the cooling rate during manufacture. While such an approach has merits, some manufacturing techniques such as the fusion process result in rapid quenching of the glass sheet from the melt and a relatively high temperature structure is “frozen in”. While some controlled cooling is possible with such a manufacturing process, it is difficult to control.